Method of fabricating a monolithic expanded beam mode electroabsorption modulator

ABSTRACT

A method of manufacturing a monolithic expanded beam mode electroabsorption modulator including a waveguide layer with a two expansion/contraction sections and an electroabsorption section arranged along a longitudinal axis. At least one patterned growth retarding layer is formed on the top surface of a substrate. The waveguide layer is formed on a portion of the top surface of the substrate by selective area growth and has an index of refraction different from the substrate. An electroabsorption portion of the waveguide layer has a thickness which is greater than thicknesses in its other portions. The semiconductor layer is formed on the waveguide layer and includes an index of refraction different from the waveguide. The waveguide and semiconductor layers are defined and etched to form the expansion/contraction and electroabsorption sections of the waveguide layer. Electrical contacts are formed, one electrically coupled to the substrate and another electrically coupled to the semiconductor layer.

[0001] This application is a divisional of U.S. patent application Ser.No. 10/056,929, filed Jan. 25, 2002, the contents of which areincorporated herein by reference.

[0002] This invention relates to semiconductor optical devices withquantum well structures. More particularly this invention relates to themonolithic integration of transparent optical mode transformers with anelectroabsorption modulator.

BACKGROUND OF THE INVENTION

[0003] A typical electroabsorption (EA) modulator is composed of asemiconductor device, which has light coupled into and out of it by twooptical fibers. The optimum optical beam profile for efficientmodulation is not the same as the optimum optical beam profile forefficient fiber coupling. This is especially true in high speed EAmodulators.

[0004] If efficient optical coupling into and out of the EA modulator isnot achieved, then system performance is degraded owing to excessiveoptical losses. Likewise, if efficient modulation is not achieved withinthe EA modulator, then system performance may be degraded owing to poorsignal quality. For optimum modulator performance, it is desirable toindependently optimize the optical beam profile in the modulation regionof the semiconductor device and at the fiber input and output couplingsurfaces of the device.

[0005] One possible solution is the inclusion of modeexpansion/contraction regions, which couple the optical signal into andout of the optical fibers with one optical beam profile, or mode, andcouple the optical signal into and out of the modulation region of thesemiconductor device with another beam mode.

[0006] There have been numerous attempts to independently optimize thesesections. One technique, described by Johnson, et. al. (U.S. Pat. No.6,162,655), uses a beam expansion technique, wherein the transfer of theoptical mode from the modulation region to an underlying passivewaveguide is through a bumped mode transfer section. The modulationregion uses quantum wells optimized for modulation properties of apreselected beam. The underlying waveguide is optimized for beamexpansion properties to allow optimum optical modes for both externalfiber coupling and modulation.

[0007] Some loss at the input and output couplings may be unavoidable,but any optical loss within an EA modulator is highly undesirable. Toavoid high optical transition loss between the waveguide and themodulation region, the thicknesses of all the layers in the transitionregion are desirably carefully controlled. This technique requires alarge number of precise fabrication steps.

[0008] Another technique for independently optimizing the modulatorregion from the beam expander region was suggested by Ido, et. al. (U.S.Pat. No. 5,742,423). The application of a “butt-joint” technique is usedto achieve independently optimized regions on the modulator. In thistechnique, the modulation region is defined through etching and the modeexpander is selectively grown. The mode transfers directly through thebutt joint region between the modulation and mode expander regions. Thistechnique has the advantage of the mode not being transferred verticallywithin the structure. The optical losses can be kept reasonably low,except for the potential of an abrupt interface with slightly differentmodal indices at the butt joint. This may cause a reflective loss if theinterface is not truly adiabatic. This technique uses regrowth ofepitaxial material on an etched structure. Epitaxial growth on etchedsurfaces can reduce yield due to possible non-uniform growth problems.Also, it can prove difficult to obtain proper mode matching betweenregions, which may lead to undesirable reflections or scattering.

[0009] Arakawa, et. al. (U.S. Pat. No. 5,757,833) disclose a selectivearea growth method to produce quantum well lasers. An integratedinfrared laser and output waveguide, fabricated by this method isdisclosed. The output waveguide is both transparent and, throughselective area growth, is shaped so as to increase the optical mode sizefor better mode coupling of the laser output to an optical fiber.Selective area growth techniques limit the absolute amount ofenhancement which can be achieved and the degree of transparencyattainable in the mode expansion section, while retaining the qualityand reliability of the device.

[0010] Lasers, such as those disclosed by Arakawa et al., must beconcerned with saturable absorber effects, which may lead tonon-linearity in the optical output power. For this and other reasonsthis technique has not widely used in laser devices. The technique ofselective area growth of quantum wells is however widely deployed tomonolithically integrate lasers with modulators where only a slightenhancement is necessary and the quality can be retained.

[0011] In addition, lasers require reflective elements for theiroperation. Arakawa et al. disclose using the cleaved surfaces of theselective growth areas as reflectors.

SUMMARY OF THE INVENTION

[0012] One embodiment of the present innovation is a monolithic singlepass expanded beam mode active optical device for light of apredetermined wavelength and a predetermined beam mode. An exemplary amonolithic single pass expanded beam mode active optical deviceincludes: a substrate; a waveguide layer coupled to the top surface ofthe substrate; a semiconductor layer coupled to the waveguide layer;first and second electrodes for receiving an electric signal coupled tothe substrate and the semiconductor layer, respectively.

[0013] The waveguide layer includes a plurality of sublayers, forming aquantum well structure, which is responsive to the electric signal. Thewaveguide layer has three sections, two expansion/contraction sectionsand an active section, which extends between and adjacent to the twoexpansion/contraction sections. At least one of the plurality ofsublayers varies in thickness within the expansion/contraction portionsof the quantum well structure. The active portion of the quantum wellstructure interacts with light of the predetermined wavelength,responsive to the electric signal. Possible interactions of the activeregion with the light include: absorption in the case of an EA modulatoror optical gain in the case of an SOA.

[0014] A further embodiment of the present innovation is a monolithicexpanded beam mode EA modulator for modulating light of a predeterminedwavelength, responsive to an electric signal. An exemplary monolithicexpanded beam mode EA modulator includes: a substrate; a waveguide layercoupled to the substrate; a semiconductor layer coupled to the waveguidelayer; and first and second electrodes for receiving the electric signalcoupled to the substrate and semiconductor layer, respectively.

[0015] The waveguide layer includes a plurality of sublayers, which forma quantum well structure. This quantum well structure includes twoexpansion/contraction sections and an electroabsorption section. Thethickness of at least one of the plurality of sublayers varies withinthe expansion/contraction sections. Also the expansion/contractionsections have a cutoff wavelength which is shorter than thepredetermined wavelength. The electroabsorption section extends between,and adjacent to the two expansion/contraction sections. The cutoffwavelength of electroabsorption section has a first value, which isshorter than the predetermined wavelength, responsive to the on-voltageof the electric signal, and has a second value, which is longer than thepredetermined wavelength, responsive to the off-voltage of the electricsignal.

[0016] Another embodiment of the present invention is method ofmanufacturing a monolithic expanded beam mode electroabsorptionmodulator of the first embodiment. The first step of this method is toform the waveguide layer on a portion of the top surface of thesubstrate by selective area growth. The waveguide layer having: awaveguide index of refraction; an electroabsorption thickness in anelectroabsorption portion of the waveguide layer that is greater thanthe thicknesses in remaining portions of the waveguide layer along thelongitudinal axis; and a plurality of sublayers forming a quantum wellstructure, each of the sublayers including a waveguide material. Next,the semiconductor layer, having a semiconductor layer index ofrefraction, is formed on the waveguide layer. Then, the waveguide layerand the semiconductor layer are defined and etched to form, along thelongitudinal axis: the electroabsorption section and the twoexpansion/contraction sections disposed on opposite sides of theelectroabsorption section. The semiconductor layer is then planarizedand first and second electrical contacts are formed on the substrate andthe semiconductor layer, respectively.

[0017] Another embodiment of the present invention is an optical signalmodulation system. An exemplary system contains: a laser to produce alight beam with a predetermined wavelength and oscillating in a firstbeam mode; an exemplary monolithic expanded beam mode EA modulator; andan optical fiber optically coupled to the monolithic expanded beam modeEA modulator and substantially optimized for low input loss andtransmission of light beams oscillating in the first beam mode.

[0018] Yet another embodiment of the present invention is an extendedrange optical communication system. In an exemplary extended rangeoptical communication system, a laser produces a light beam with apredetermined wavelength and a first beam mode. This light beam isoptically coupled at the input end and transmitted along a first opticalwaveguide. The output end is optically coupled to a monolithic expandedbeam mode optical amplifier. An exemplary monolithic expanded beam modeoptical amplifier includes: an input surface substantially optimized forlow input loss of light beams with the first beam mode; an expansionsection to expand the beam mode of the light beam for increasedconfinement of the light beam; an optical amplification section, whichincludes a semiconductor gain medium for amplifying light of thepredetermined wavelength; a contraction section to contract the beammode of the light beam to about the first beam mode; and an outputsurface. The amplified light beam is optically coupled a second opticalwaveguide, which is substantially optimized for low input loss andtransmission of light beams with the first beam mode.

[0019] Another exemplary embodiment of the present invention is alow-loss demultiplexer for demultiplexing a plurality of temporallyoffset channels, each of which is modulated at a channel bit rate andtemporally offset from the remaining channels by less than a minimumtime between bits. The input optical signal source is coupled into amonolithic expanded beam mode-EA modulator which may be periodicallymodulated at the channel bit rate with the temporal offset of onechannel of the input signal to select that channel. The resulting singlechannel signal is then optically coupled to a receiver.

[0020] Yet another exemplary embodiment of the present invention is anexemplary low-loss demultiplexer for demultiplexing a time divisionmultiplexed (TDM) optical signal have a plurality of channels, eachchannel transmitted as blocks which are temporally interleaved withblocks of other channels. The exemplary low-loss demultiplexer includes:an optical beam splitter for splitting the TDM signal; a monolithicexpanded beam mode EA modulator to select blocks of a single channel;and a buffer optically coupled to the output surface of the monolithicexpanded beam mode electroabsorption modulator to store the selectedblocks.

BRIEF DESCRIPTION OF THE FIGURES

[0021]FIG. 1A is a top plan drawing of an exemplary monolithic expandedbeam mode device according to the present invention.

[0022]FIG. 1B is a side cut-away drawing of an exemplary monolithicexpanded beam mode device according to the present invention.

[0023]FIG. 1C is a front cut-away drawing of an exemplary monolithicexpanded beam mode device according to the present invention.

[0024]FIG. 2 is a flowchart illustrating an exemplary method ofmanufacture of the monolithic expanded beam mode device of FIGS. 1A-1C.

[0025]FIGS. 3A, 4A, and 5A are top plan drawings of an exemplarymonolithic expanded beam mode device during manufacture according to theflowchart of FIG. 2.

[0026]FIGS. 3B, 4B, and 5B are side cut-away drawings of an exemplarymonolithic expanded beam mode device during manufacture according to theflowchart of FIG. 2.

[0027]FIGS. 3C, 4C, and 5C are front cut-away drawings of an exemplarymonolithic expanded beam mode device during manufacture according to theflowchart of FIG. 2.

[0028]FIG. 6 is a flowchart illustrating an exemplary method ofmanufacture of an alternative exemplary monolithic expanded beam modedevice according to the present invention.

[0029]FIGS. 7A, 8A, 9A, and 10A are top plan drawings of an exemplarymonolithic expanded beam mode device during manufacture according to theflowchart of FIG. 6.

[0030]FIGS. 7B, 8B, 9B, and 10B are side cut-away drawings of anexemplary monolithic expanded beam mode device during manufactureaccording to the flowchart of FIG. 6.

[0031]FIG. 11A is a side plan drawing of an alternative exemplarymonolithic expanded beam mode device fabricated according the flowchartof FIG. 6.

[0032]FIG. 11B is a front plan drawing of an alternative exemplarymonolithic expanded beam mode device fabricated according the flowchartof FIG. 6.

[0033]FIG. 12A is a graph illustrating absorption as a function ofwavelength for an exemplary unstrained quantum well structure.

[0034]FIG. 12B is a graph illustrating absorption as a function ofwavelength for an exemplary strained quantum well structure.

[0035]FIG. 13 is a block diagram illustrating an exemplary extendedrange optical communications system.

[0036]FIGS. 14A and 14B are block diagrams illustrating an exemplarydemultiplexer for temporally offset signals.

[0037]FIG. 15 is a band diagram illustrating an exemplary strain quantumwell structure.

[0038]FIG. 16 is a block diagram illustrating an exemplary demultiplexerfor TDM signals.

[0039]FIG. 17A is a top plan drawing of an exemplary multi-devicemonolithic multiplexer/demultiplexer according to the present invention.

[0040]FIG. 17B is a side cut-away drawing of an exemplary multi-devicemonolithic multiplexer/demultiplexer according to the present invention.

[0041]FIG. 17C is a front cut-away drawing of an exemplary multi-devicemonolithic multiplexer/demultiplexer according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0042] One embodiment of the present invention is a monolithic expandedbeam mode EA modulator which includes two mode expansion/contraction(E/C) sections and a modulation section formed in a single piece byselective area growth. Additional embodiments include a method ofmanufacturing, and exemplary uses of a device of this type.

[0043] Desirably, an optical modulator waveguide would be designed tohave a large optical mode on the input to the chip for optical couplingwith minimal transmissive and reflective losses, followed by a tightlyconfining modulation region to achieve good modulation efficiency, andending with a large optical output mode to couple to the output fiberwith low transmissive and reflective losses. The modal properties ofoptical waveguides are a function of waveguide thickness. It can beshown that when an optical waveguide becomes thinner, the modalconfinement in the waveguide decreases. As described below, thisproperty may be used to design a monolithic expanded beam mode EAmodulator.

[0044] FIGS. 1A-C illustrate an exemplary embodiment of the inventivemodulator. FIG. 1A is a top plan view of the exemplary monolithicexpanded beam mode EA modulator 100. Cut line 102 shows the orientationof the side cut-away view of modulator 100 shown in FIG. 1B and cut line104 shows the orientation of the front cut-away view of modulator 100shown in FIG. 1C.

[0045] Exemplary monolithic expanded beam mode EA modulator 100 isformed with three layers: substrate layer 106; waveguide layer 108; andsemiconductor layer 110. Waveguide layer 108 may contain a number ofsublayers, as shown. These sublayers form a quantum well structurewithin this layer. This structure may include a single quantum well,multiple quantum wells, separate confinement layers or a bulk activematerial. Substrate layer 106 and semiconductor layer 110 may alsocontain a plurality of sublayer. Additionally, semiconductor layer 110may desirably function as a cladding layer as well as the p-typematerial of the P-I-N quantum well structure.

[0046] A modulator desirably has a waveguide profile similar to that ofwaveguide layer 108 in FIG. 1B. It is desirably thin at the input/outputsurfaces 116 of the modulator chip and thicker in the modulation section112. The quantum well structure in modulation section 112 is designed toprovide modulation at a predetermined working wavelength.

[0047]FIGS. 12A and 12B (prior art) illustrate how quantum well EAmodulators operate. The graph in FIG. 12A represents an exemplary EAmodulator with unstrained quantum well layers and the graph in FIG. 12Brepresents an exemplary EA modulator with strained quantum well layers.Both are designed to operate at working wavelength 1214. Curves 1200 inthe two graphs represent normalized absorption spectra for these quantumwell structures with no applied field and curves 1202 representnormalized absorption spectra with a bias voltage applied. As shown inthese graphs application of a bias voltage, which may be as small as2-10 V, moves the absorption peak 1212 to the working wavelength 1214.This transition is extremely fast and modulation rates are generallylimited only by the rate at which the bias voltage may be modulated.Signals having bit rates exceeding 40 GHz are possible with such amodulator.

[0048] The quantum well structure is desirably designed to maximize theon/off ratio, the normalized height of the biased absorption peak at theworking wavelength. Additionally, on/off ratio 1210 representing anexemplary strained quantum well structure may be designed, as shown inFIG. 12B, to be greater than on/off ratio 1208 of a similar, butunstrained, exemplary quantum well structure, as shown in FIG. 12A.

[0049] The exemplary operation of EA modulators illustrated in FIGS. 12Aand 12B involves modulation between an unbiased, ‘on-voltage’, and abiased, ‘off-voltage’, state. It is contemplated, though, that thequantum well structure may be designed differently, allowing an offsetof these voltages. For example, on-voltage may be negative andoff-voltage unbiased, or both may be positive voltages, as long as thespectral shift of the absorption spectrum provides an adequate on/offratio. Another important consideration is that cutoff wavelength, thelongest wavelength at which significant absorption occurs, may beshorter than the working wavelength for the on-voltage to ensuresubstantial transparency of the modulator when it is allowing light topass. Temperature may also affect the device performance.

[0050] An exemplary expanded beam mode EA modulator which has a workingwavelength of 1.55 μm, such as shown in FIGS. 1A-1C, may have a band gapin modulation section 112 which corresponds to an absorption peak of1.51-1.53 μm. This band gap allows the device to efficiently absorblight when the off-voltage is applied and to pass light through withlittle absorption when biased at the on-voltage. There is generally someabsorption of the working wavelength, when the device is biased at theon-voltage. This absorption amount is a tradeoff in designing thedevice.

[0051] It is possible to form an EA modulator that has a cutoffwavelength sufficiently short to eliminate substantially all absorptionat the on-voltage bias, but this may require an undesirably largevoltage difference for modulation. One may, however, form passivewaveguides that have such a cutoff wavelength in E/C sections 114. Aproperty of quantum wells which may be exploited to assist with thisissue is that, as the thickness of the quantum well increases, the bandgap or energy of the absorption peak decreases. This corresponds to asignificant decrease in the cutoff wavelength of light absorbed by thequantum well structure.

[0052] By using selective area growth it is possible to grow a singlemulti-layer quantum well structure of varying thickness, and thus havinga varying cutoff wavelength. Therefore, the thickness of waveguide layer108 can easily be modified through the use of selective area growth,which is described below with reference to FIG. 2. This allows the bandgap of the E/C sections 114 to be increased, by decreasing the thicknessof the sublayers. Increasing the band gap of the quantum wells in thesesections effectively makes the quantum wells transparent to the workingwavelength for both the on and the off-voltage. The use of selectivearea growth techniques allows the thickness of E/C sections 114 to varyfrom a minimum at input/output surfaces 116 to a maximum at thethickness of modulator section 112. The entire waveguide layer 108 maybe formed as a single unit, the sublayers of the quantum well structurestretching continuously from one I/O surface 116 to the other. Usingselective area growth techniques, E/C sections of 75 μm, or longer, maybe formed, which have a continuously varying thickness. The thicknessvariation in the E/C sections may be desirably gentle enough to allowthe beam modes to expand and contract adiabatically. This adiabaticexpansion and contraction, coupled with the monolithic construction,diminishes the possibility of scattering losses within exemplaryexpanded beam mode EA modulator 100.

[0053] The structure of both the quantum wells and the thickness profilemay be closely controlled. Enhancements of 2.5 times in the thickness ofmodulator section 112 over E/C section 114 may be achieved. The designedenhancement is desirably sufficient to maintain an absorption peak inthe bulk of the mode E/C section far enough from the working wavelengthto ensure substantial transparency, for example, 40 nm or more from theworking wavelength.

[0054] For an exemplary 1.55 μm expanded beam EA modulator, modulationsection 112 may be designed with an on-voltage absorption peak,corresponding to peak 1212 in FIG. 12B, of 1.52-1.53 μm and E/C section144 may be designed with an on-voltage absorption peak of <1.51 μmsubstantially throughout. For an exemplary 1.32 μm expanded beam EAmodulator, modulation section 112 may be designed with an on-voltageabsorption peak of 1.29-1.30 μm and expansion/contraction section 144may be designed with an absorption peak of <1.28 μm substantiallythroughout.

[0055]FIG. 2 is a flowchart describing an exemplary selective areagrowth technique for producing exemplary expanded beam mode EA modulator100 from FIG. 1A-1C. FIGS. 3A-C, 4A-C, and 5A-C illustrate various stepsof this exemplary fabrication process.

[0056] The process begins with a substrate, step 200. Substrate 106,shown in FIG. 3, may function as both a cladding layer to assist incontainment of the beam in the device and as the N layer of the P-I-Nquantum well structure. (Although this description assumes that thesubstrate is the N side of the P-I-N structure, one skilled in the artwill understand that the substrate could be the P side with thesemiconductor layer 110 formed of N-type material instead.) Thesubstrate is preferably formed of a III/V semiconductor, such as InP,GaAs, or InGaAsP. The substrate may also be formed of multiple layerssuch as GaAs grown on silicon or alumina.

[0057] A patterned growth-retarding layer is formed on the top surfaceof the substrate, step 202. Materials which retard growth of III/Vmaterials, such as SiN or SiO₂, make up the growth-retarding layer. Thegrowth-retarding layer may be formed and patterned using any standardtechniques known in the semiconductor industry. FIGS. 3A-C show thewafer at this point in the fabrication process. Patternedgrowth-retarding layer 300 is shown in FIG. 3A as two rectangularregions with a channel between disposed along longitudinal axis 102(also the cut line for the cutaway view in FIG. 3B). For an exemplaryexpanded beam EA modulator 2 μm wide, a 15 to 20 μm channel is desirableto provide substantial flatness of the layers in a transverse direction.Depending on the profile desired for the waveguide layer, otherpatterns, such as paired trapezoids or triangles, may be used. A largernumber of regions may also be used.

[0058] Next a plurality of sublayers making up the waveguide layer aregrown, step 202. Metal organic chemical vapor deposition (MOCVD) is thepreferred method for deposition of the waveguide sublayers, but otherepitaxial deposition techniques may also be employed, such as molecularbeam epitaxy (MBE) and-chemical beam epitaxy (CBE). Near thegrowth-retarding regions the growth rate is enhanced owing to gas phasediffusion and surface diffusion of the reactants in the MOCVD reactoraway from growth-retarding regions 300. The quantum wells layers thusdeposited 400, as shown in FIGS. 4A-C, are made thicker in themodulation section 402 of the device owing to the growth-retardingmasks. An exemplary quantum well structure, for use with typical opticalcommunication signals, may be designed have an unbiased absorption peakapproximately 0.01 μm longer in the central modulation section than inmode expansion sections 404. For example, a 1.55 μm modulator maydesigned such that the quantum wells attain an absorption peak at1.52-1.53 μm in central region 402, and a peak <1.51 μm in the modeexpansion sections 404.

[0059] The quantum wells and barriers are preferably composed ofIn_(x)Ga_((1-x))As_(y)P_((1-y)) materials as well asIn_(x)Al_(y)Ga_((1-x))As_((1-y)) and In_(x)Ga_((1-x))As materials.Specific selections of x and y are dependent on the desired bandgap andstrain, if any, desired. These sublayers may also be formed by otherpermutations of alloys formed from these elements. The quantum wells andbarriers desirably have a sufficiently larger refractive index than thatof substrate 106 so that the quantum wells and barriers act as awaveguide.

[0060] Next a cladding layer is formed over the waveguide layer, step206. This step of the fabrication process is illustrated in FIG. 5A-C.Preferably, cladding layer 500 is formed using the same method as thewaveguide layer. The cladding layer desirably has a refractive indexlower than waveguide layer 400, preferably similar to that of substrate106, to ensure light containment. Additionally, the cladding layer maybe formed of a P type material, preferably P-type InP or GaAs. Also, thecladding may be formed in multiple sublayers.

[0061] Step 208 defines the mesa structure of the expanded beam mode EAmodulator. The mesa includes the EA modulation section and two E/Csections of the waveguide and cladding layers. This mesa may bestraight, as shown in FIGS. 1A and 1C, or laterally tapered to furtherenhance mode coupling into the fiber. Next these layers are etched toform the mesa structure, step 210, and growth-retarding layer 300 isremoved, step 212. Although step 212 is shown following step 210 in FIG.2, it is contemplated that step 212 could alternatively take placebetween steps 204 and 206 or after any of steps 214, 216, 218, or 220.Additionally, step 212 could be skipped entirely if the growth-retardinglayers do not interfere with the operation of expanded beam mode EAmodulator 100.

[0062] Once the mesa is formed, the cladding layer is planarized, step214, p and n type ohmic contacts are deposited on the cladding layer andsubstrate layer respectively, and the device may be cleaved to formexemplary expanded beam mode EA modulator 100 illustrated in FIGS. 1A-C.Steps 208, 210, 212, 214, 216, 218, and 220 may be carried out by any ofa number of standard semiconductor fabrication techniques known to thoseskilled in the art.

[0063]FIG. 15 shows one possible band gap diagram of a single quantumwell structure in which both the group III and group V components arechanged, which may be employed in an exemplary expanded beam mode EAmodulator according to the present invention. The composition of thisexemplary well structure is linearly varied for both of the group IIIand V from In_(0.380)Ga_(0.620)As (strain: −1.0%) on the n-InP substrateside to In_(0.490)Ga_(0.510)As_(0.962)P_(0.038) (strain: −0.4%) on thep-region side (also InP). This exemplary structure yields a bandgapwavelength of about 1.51 μm.

[0064]FIGS. 11A and 11B illustrate another exemplary expanded beam modeEA modulator 1100. Exemplary expanded beam mode EA modulator 1100includes a recessed I/O window 802 and anti-reflection (AR) coating1104. Recessing the I/O surfaces of waveguide layer 108 withinsemiconductor layer 110 may provide improved light coupling by reducingreflections, and may reduce fabrication losses during cleaving. ARcoating 1104 improves light coupling by reducing reflections.

[0065]FIG. 6 is a flowchart including two addition alternative steps inthe expanded beam mode EA modulator fabrication process of FIG. 2. Thesetwo alternative fabrication steps may be used to provide the recessedI/O window 802 and anti-reflection (AR) coating 1104, illustrated inFIGS. 11A and 11B. This method of manufacture is otherwise identical tothat just described. FIGS. 7A-B, 8A-B, 9A-B, 10A-B, and 11A-B followthis process. In FIGS. 7A-B, 8A-B, 9A-B, and 10A-B illustrate theformation of two exemplary expanded beam mode EA modulators formed sideby side on a wafer.

[0066]FIGS. 7A and 7B illustrate the fabrication process after steps200, 202, and 204 in FIG. 6. Patterned growth-retarding regions 300 andwaveguide layer 400 are shown deposited on substrate 106 as in FIGS.4A-C. Next the waveguide layer 400 is etched to expose substrate 106,step 600, forming windows for I/O surfaces 802. FIGS. 8A and 8Billustrate this step. These I/O surfaces may be defined and etched byany standard semiconductor fabrication method which produces asufficiently planar surface.

[0067] The process of FIG. 6 is then continued as in FIG. 2 throughsteps 206, 208, 210, 212, 214, 216, and 218. FIGS. 9A and 9B illustratesthe device in process following step 206 and FIGS. 10A and 10B show thedevice in process following step 214. FIGS. 10A and 10B show anexemplary position for cleavage line 1000 for step 220, cleaving thedevice. This line desirably falls within the window between recessed I/Osurfaces 802.

[0068] The final step, step 602 in FIG. 6 is the deposition of ARcoating 1104 on the cleaved surfaces. This deposition may beaccomplished by a number of methods known to those skilled in the art,such as vapor phase deposition or sputtering.

[0069] It is noted that either of the alternative processing steps shownin FIG. 6, steps 600 and 602, may be added individually to the morebasic exemplary process shown in FIG. 2.

[0070] Another embodiment of the present invention is a monolithicexpanded beam mode semiconductor optical amplifier (SOA). An SOA may beformed as a wave-guide structure that includes a semiconductor gainmedium which may be bulk or a quantum well structure. The SOA operatesas a traveling wave amplifier and may be used to increase the outputpower of a laser. In an optical communication system, an SOA may be usedto boost a weakened optical signal along an extended fiber. In this way,the distance over which an optical signal may be transmitted withouthaving to be received and re-transmitted can be increased. It isdesirable to confine as much of the optical signal within the gainmedium as possible to provide efficient amplification. Because anycoupling losses will also reduce the effective gain of an SOA, it isobviously desirable to optimize the optical coupling of the SOA as well.

[0071] Therefore, an SOA, like an optical modulator, would desirably bedesigned to have a large optical mode on the input to the chip for goodoptical coupling, followed by a tightly confining amplification regionto achieve efficient amplification, and ending with a large opticaloutput mode to couple to the output fiber with low loss. An exemplarymonolithic expanded beam mode SOA may be produced by the exemplaryprocesses of FIGS. 2 or 6. Such a device is similar to the previouslydescribed monolithic expanded beam mode EA modulator of either FIGS.1A-C or FIGS. 11A-B with the difference being in waveguide layer 108(which are grown in step 204 of the processes). Specifically, this layermay be formed of either a plurality of sublayers forming a quantum wellstructure or a bulk gain material. If waveguide layer 108 is formed as aquantum well structure, then in section 112 of FIGS. 2A-B and 11 thethickness and composition of the waveguide sublayers are substantiallyoptimized to provide a gain medium, rather than a tunable absorptionmedium quantum well structure. Also, the inclusion of AR coating 1104may be desirable for a monolithic expanded beam SOA to reduce thepossibility of oscillation.

[0072] The range over which an optical signal can be transmitted in sucha system is limited by losses within optical fibers. Efficientmodulation of the signal with low loss and a high signal to noise ratiocan help, but eventually fiber losses render the signal undetectable.One solution is to detect the signal before it becomes undetectable andthen retransmit the signal. This slows the overall transmission speed ofthe system and may introduce errors. FIG. 13 illustrates an exemplaryextended range fiber optic communication system, which employs amonolithic expanded beam mode EA modulator, modulator 1302, to provideefficient modulation, and a monolithic expanded beam mode SOA, amplifier1306, to extend the range between retransmissions.

[0073] Laser 1300, which provides input light to the system, may beeither a CW or mode-locked laser, but is preferably a fiber-coupled, CWdiode laser. The light from laser 1300 is coupled into monolithicexpanded beam mode EA modulator 1302, which modulates the light tocreate the input signal for the system. This signal is then coupledinto, and transmitted through, first optical fiber 1304, preferably alow-loss single mode fiber. The signal is next coupled into monolithicexpanded beam mode SOA 1306, where the signal is amplified to compensatefor losses which have occurred during transmission. The amplified signalis then coupled into, and transmitted through, second optical fiber1308. The signal is finally detected by detector 1310.

[0074] In this way, the range at which a signal may be transmittedthrough the fiber optic communication system, without having to detectthe signal and retransmit it, may be increased. Additional amplifier andoptical fiber stages may be added to further extend the range of thefiber optic communication system. Although the preferred system wouldemploy both a monolithic expanded beam mode EA modulator and amonolithic expanded beam mode SOA's designed according to the presentinvention, either may be used alone within such a system.

[0075] Another feature of optical communications systems is the abilityto multiplex a number of signals and simultaneously transmit thesesignals along the same optical fiber. To realize this advantage anoptical communications system needs a method of multiplexing anddemultiplexing the signals. As with other components in an opticalcommunications system, it is desirable that the multiplexers anddemultiplexers operate efficiently with low loss. In the case oftemporally multiplexed signals, which contains a number of separatesignals each temporally offset to modulate out of phase with the othersignals, it is also desirable for the demultiplexer to operate at highspeed to maintain a high bit rate for the signals.

[0076] Another exemplary embodiment of the present invention,illustrated in FIGS. 14A and 14B, is the use of a monolithic expandedbeam mode EA modulator, constructed according to the present invention,as a demultiplexer for temporally multiplexed optical signals. As shownin FIG. 14A, temporally multiplexed signal 1400 is coupled intomonolithic expanded beam mode EA modulator 1402. Modulator 1402 issynchronized with one of the individual signals which make upmultiplexed signal 1400 to transmit only the portions of multiplexedsignal 1400 which make up the selected signal. The resulting output isdemultiplexed signal 1404. FIG. 14B illustrates a complete demultiplexerfor demultiplexing two temporally multiplexed signals. First multiplexedsignal 1400 is split into two beams by splitter 1406. The beams, stillcontaining the multiplexed signals, are then coupled into monolithicexpanded beam mode EA modulators 1402 and 1408. Modulator 1402 issynchronized to first demultiplexed signal 1402 and modulator 1408 issynchronized to second demultiplexed signal 1410. Temporaldemultiplexers for multiplexed signals containing 3 or more signals maybe constructed in a similar manner.

[0077]FIG. 16 illustrates another exemplary embodiment of the presentinvention as a demultiplexer for a time division multiplexed (TDM)optical communication system. The exemplary demultiplexer is illustratedfor a four compressed channels (labeled A, B, C, and D), but it is notedthat other numbers of channels may be demultiplexed in a similar manner.

[0078] In this exemplary demultiplexer, TDM signal 1600 is coupled tobeam splitter 1602 which splits the signal into four substantiallyidentical signals. The split signals are fed into monolithic expandedbeam mode EA modulators 1604, 1606, 1608, and 1610. These fourmodulators are operated to transmit only blocks containing informationfor a single compressed channel. For example, modulator 1604 transmitsonly blocks of compressed channel A, signal 1612, and stops the blocksof the other channels. Modulators 1606, 1608, and 1610, likewise,transmit compressed single channel signals 1614, 1616, and 1618,respectively. The four compressed single channel signal are then eachloaded into a buffer: compressed channel A signal 1612 into channel Abuffer 1620; compressed channel B signal 1614 into channel B buffer1622; compressed channel C signal 1616 into channel C buffer 1624; andcompressed channel D signal 1618 into channel D buffer 1626. Thecompressed signals stored in these buffers are then decompressed andspliced to be transmitted as single channel signals 1628 (channel A),1630 (channel B), 1632 (channel C), and 1634 (channel D). It isdesirable for the single channel signals to be decompressed enough thattheir blocks may be spliced into a continuous signal, but it is possiblethat the signals may not be completely decompressed by the buffer.

[0079] It is noted that, by using the selective area growth processesdescribed above, a number of coupled expanded beam mode active opticaldevices may be monolithically fabricated together with adiabatic beammode conversion E/C sections to reduce coupling losses. Passive opticalcomponents, such as a waveguide beam splitter, may also bemonolithically integrated in the same way. For example an SOA sectioncould be added in front of the EA modulation section to provide astronger modulated signal. A monolithic amplified modulator of this typemay be particularly useful in a demultiplexer application, such as thosedescribed above.

[0080] FIGS. 17A-C illustrate exemplary multi-device monolithicmultiplexer/demultiplexer (mux/demux) 1700. Cut lines 1702 and 1704 inthe top plan drawing of FIG. 17A show the position and orientation ofthe cut-away drawings of FIGS. 17B and 17C, respectively. Exemplarymulti-device monolithic mux/demux 1700 is formed in three layers:substrate 1706; waveguide layer 1708, and semiconductor layer 1710; asthe single devices described above.

[0081] When used as a demultiplexer, the multiplexed signal is coupledinto exemplary multi-device monolithic mux/demux 1700 at I/O surface1720. The input multiplexed signal is then split in waveguide beamsplitter 1718 forming two desirably identical multiplexed signals. Thebeam modes of these two multiplexed signals are expanded as they passthrough separate E/C sections 1716 and 1717. The multiplexed signals aremodulated in EA modulation sections 1714 and 1715 to transmit adifferent one of the constituent signal from each. The beam modes of thedemultiplexed signals are contracted in E/C sections 1712 and 1713 forefficient optical coupling through I/O surfaces 1722 and 1723.

[0082] When used as a multiplexer, separate light beams are coupled intoexemplary multi-device monolithic mux/demux 1700 at I/O surfaces 1722and 1723. The beam modes of these two input light beams are expanded asthey pass through separate E/C sections 1712 and 1713. These input lightbeams are then separately modulated in EA modulation sections 1714 and1715 to transmit separate constituent signal from each. The beam modesof the two constituent signals are contracted in E/C sections 1716 and1717. The two constituent signals are combined in waveguide beamsplitter 1718 forming a multiplexed signal. The contracted beam mode ofthe multiplexed signal is desirably optimized for efficient opticalcoupling through I/O surface 1720.

[0083] The device illustrated in FIGS. 17A-17C is designed to multiplexand demultiplex time domain multiplexed signals, such as temporallyoffset signals or TDM signals. It is noted that the two EA modulatorsections 1714 and 1715 may be optimized to different wavelengthsallowing the device to be used for wavelength multiplexing as well. Itis also noted that, although illustrated exemplary multi-devicemonolithic mux/demux 1700 is designed for multiplexed signals having twoconstituent signals, a similar device may be constructed for multiplexedsignals having a larger number of constituent signals.

[0084] Although the embodiments of the invention described above havebeen in terms of EA modulators and SOA's, it is contemplated thatsimilar concepts may be practiced with other optical components. Also,it will be understood to one skilled in the art that a number of othermodifications exist which do not deviate from the scope of the presentinvention as defined by the appended claims.

What is claimed:
 1. A method of manufacturing a monolithic expanded beammode electroabsorption modulator which includes a substrate with a topsurface and substrate index of refraction; a waveguide layer with a twoexpansion/contraction sections and an electroabsorption section arrangedalong a longitudinal axis; and a semiconductor layer, the methodcomprising the steps of: a) forming at least one patterned growthretarding layer on the top surface of the substrate; b) forming awaveguide layer having a waveguide index of refraction different fromthe substrate index of refraction on a portion of the top surface of thesubstrate by selective area growth, the waveguide layer including anelectroabsorption portion having an electroabsorption thickness which isgreater than thicknesses in other portions of the waveguide layer; c)forming the semiconductor layer on the waveguide layer, thesemiconductor layer including a semiconductor layer index of refractiondifferent from the waveguide index of refraction; d) defining andetching the waveguide layer and the semiconductor layer to form the twoexpansion/contraction sections and the electroabsorption section of thewaveguide layer; e) forming a first electrical contact electricallycoupled to the substrate; and f) forming a second electrical contactelectrically coupled to the semiconductor layer.
 2. The method of claim1, wherein forming the at least one patterned growth retarding layer instep a) includes forming a plurality of growth retarding elements, thegrowth retarding elements defining a channel extending along a centralportion of the longitudinal axis, wherein the channel has a widthgreater than an electroabsorption width of the electroabsorptionsection.
 3. The method of claim 1, wherein step d) further includes thestep of removing the growth-retarding layer.
 4. The method of claim 1,wherein step d) further includes the step of planarizing thesemiconductor layer.
 5. The method of claim 1, wherein forming the atleast one patterned growth retarding layer in step a) includes at leastone of: sputtering growth retarding material onto the substrate;depositing the growth retarding material onto the substrate byvaporization deposition; depositing the growth retarding material ontothe substrate by evaporation deposition; or epitaxially growing thegrowth retarding material on the substrate.
 6. The method of claim 1,wherein forming the waveguide layer in step b) includes epitaxiallygrowing a plurality of sublayers to form a quantum well structure withinthe waveguide layer, each of the sublayers including waveguide material.7. The method of claim 1, wherein forming the waveguide layer in step b)includes at least one of: sputtering waveguide material onto thesubstrate; depositing the waveguide material onto the substrate byvaporization deposition; depositing the waveguide material onto thesubstrate by evaporation deposition; or epitaxially growing thewaveguide material on the substrate.
 8. The method of claim 1, whereinforming the semiconductor layer in step c) includes at least one of:sputtering semiconductor material onto the waveguide layer; depositingthe semiconductor material onto the waveguide layer by vaporizationdeposition; depositing the semiconductor material onto the waveguidelayer by evaporation deposition; or epitaxially growing thesemiconductor material on the waveguide layer.
 9. The method of claim 1,further comprising: g) cleaving the substrate, the waveguide layer, andthe semiconductor layer to form an input/output surface on eachexpansion/contraction section of the waveguide layer.
 10. The method ofclaim 9, further comprising: h) depositing an anti-reflection coating onat least one of the input/output surfaces.
 11. A method of manufacturinga monolithic expanded beam mode electroabsorption modulator whichincludes a substrate with a top surface and substrate index ofrefraction; a waveguide layer with a two expansion/contraction sectionsand an electroabsorption section arranged along a longitudinal axis; anda semiconductor layer, the method comprising the steps of: a) forming atleast one patterned growth retarding layer on the top surface of thesubstrate; b) forming a waveguide layer having a waveguide index ofrefraction different from the substrate index of refraction on a portionof the top surface of the substrate by selective area growth, thewaveguide layer including an electroabsorption portion having anelectroabsorption thickness which is greater than thicknesses in otherportions of the waveguide layer; c) defining and etching the waveguidelayer to form the two expansion/contraction sections and theelectroabsorption section; d) forming the semiconductor layer on thewaveguide layer and an exposed portion of the top surface of thesubstrate, the semiconductor layer including a semiconductor layer indexof refraction different from the waveguide index of refraction; e)forming a first electrical contact electrically coupled to thesubstrate; and f) forming a second electrical contact electricallycoupled to the semiconductor layer.
 12. The method of claim 11, whereinforming the at least one patterned growth retarding layer in step a)includes forming a plurality of growth retarding elements, the growthretarding elements defining a channel extending along a central portionof the longitudinal axis, wherein the channel has a width greater thanan electroabsorption width of the electroabsorption section.
 13. Themethod of claim 11, wherein forming the at least one patterned growthretarding layer in step a) includes at least one of: sputtering growthretarding material onto the substrate; depositing the growth retardingmaterial onto the substrate by vaporization deposition; depositing thegrowth retarding material onto the substrate by evaporation deposition;or epitaxially growing the growth retarding material on the substrate.14. The method of claim 11, wherein forming the waveguide layer in stepb) includes epitaxially growing a plurality of sublayers to form aquantum well structure within the waveguide layer, each of the sublayersincluding waveguide material.
 15. The method of claim 11, whereinforming the waveguide layer in step b) includes at least one of:sputtering waveguide material onto the substrate; depositing thewaveguide material onto the substrate by vaporization deposition;depositing the waveguide material onto the substrate by evaporationdeposition; or epitaxially growing the waveguide material on thesubstrate.
 16. The method of claim 11, wherein step c) further includesthe step of removing the growth-retarding layer.
 17. The method of claim11, wherein: step c) further includes the step of etching the waveguidelayer to form an input/output surface on each expansion/contractionsection of the waveguide layer; and step d) further includes the step offorming the semiconductor layer on the waveguide layer so as to coat theinput/output surfaces.
 18. The method of claim 17, further comprising:g) cleaving the substrate and the semiconductor layer to form a recessedinput/output window on each expansion/contraction section of thewaveguide layer.
 19. The method of claim 18, further comprising: h)depositing an anti-reflection coating on at least one of theinput/output surfaces.
 20. The method of claim 11, wherein step d)further includes the step of planarizing the semiconductor layer. 21.The method of claim 11, wherein forming the semiconductor layer in stepd) includes at least one of: sputtering semiconductor material onto thewaveguide layer; depositing the semiconductor material onto thewaveguide layer by vaporization deposition; depositing the semiconductormaterial onto the waveguide layer by evaporation deposition; orepitaxially growing the semiconductor material on the waveguide layer.22. The method of claim 11, further comprising: g) cleaving thesubstrate, the waveguide layer, and the semiconductor layer to form aninput/output surface on each expansion/contraction section of thewaveguide layer.
 23. The method of claim 22, further comprising: h)depositing an anti-reflection coating on at least one of theinput/output surfaces.